Cancer chemotherapy involves the administration of one or more cytotoxic or cytostatic drugs to a cancer sufferer. The goal of chemotherapy is to eradicate a substantially clonal population (colony) of transformed cells from the body of the individual, or to suppress or to attenuate growth of the colony, which is most commonly referred to as a tumor. Tumors may occur in solid or liquid form, the latter comprising a cell suspension in blood or another body fluid. A secondary goal of chemotherapy is stabilization (clinical management) of the afflicted individual's health status. Although the tumor may initially respond to chemotherapy by, e.g., stabilizing or reducing its growth rate, in many instances the initial chemotherapeutic treatment regimen becomes less effective or ceases to impede tumor growth. Conventional treatment regimes endorse the use of additional or substitute chemotherapeutic drugs, including drug combinations, in an effort to regain control over tumor growth. However, it is well known that transformed cells in a tumor may acquire resistance to a broad spectrum of chemotherapeutic drugs, including drugs to which the tumor has not hitherto been exposed during treatment. This acquisition of a multidrug-resistant (or multidrug-resistance) phenotype significantly constrains the chemotherapeutic choices available to the clinician, and significantly worsens prognosis for the afflicted individual. Acquisition of multidrug resistance is particularly problematic in carcinomas originating in secretory epithelia, including lung, gastrointestinal tract, mammary, reproductive tract, endocrine and neuroendocrine epithelia.
Tumor cell transformation is the process by which a cell escapes normal control mechanisms governing the cell's tissue-specific phenotype and differentiation state. Thus, transformation often involves "dedifferentiation," which is defined as an inappropriate return to a less committed or less tissue-specific phenotype. Alternatively, transformation involves incomplete or arrested differentiation of cells normally responsible for replenishing cells lost to normal tissue turnover. Transformed cells of epithelial origin produce tumors that are carcinoma cell colonies (carcinomas). When in a gland-like configuration or derived from secretory tissue, such epithelium-derived tumors are referred to as adenocarcinomas. In contrast, transformed cells of mesenchymal origin produce tumors that are sarcoma cell colonies (sarcomas). Transformed cells of the hematopoietic lineage produce leukemias, lymphomas or lymphosarcomas, each of which often occur as cell suspension tumors. In contrast, the primary tumor growth of a carcinoma or sarcoma usually remains near the site of initial cell transformation. However, secondary foci (metastases) of tumor growth can arise at other sites, which can be far removed from the primary tumor growth site. The presence and/or abundance of metastases indicates the degree to which transformed cells have strayed from their normal tissue-specific phenotype and/or acquired invasive properties.
Phenotypically, cell transformation involves the display of altered or abnormal structural (e.g. antigenic) and functional cellular properties. These altered properties provide the transformed cell with a survival or growth advantage over neighboring, non-transformed cells in its tissue of origin. The advantage may arise from acquisition of autocrine growth regulation, abnormal activation of genes controlling or regulating the cell division cycle, abnormal suppression of genes needed for normal exit from or arrest of the cell division cycle, or other changes affecting cell growth and/or survival. Over time, divisions of the transformed cell produce a colony (tumor) of daughter cells each having the phenotypic advantage gained by the original transformed cell. The imposition of chemotherapy subjects the tumor to selection pressure, in effect encouraging further phenotypic change by which tumor cells may escape the cytotoxic effects of a chemotherapeutic drug. Thus, the structural and functional properties of transformed cells in a tumor can fluctuate over time and over the course of chemotherapeutic treatment.
A significant survival advantage is associated with the acquisition of a multidrug-resistance phenotype, which arises from expression of a cellular gene encoding a protein that removes diverse chemotherapeutic drugs or drug metabolites from the intracellular milieu. Drug export diminishes cytotoxic effect, thereby protecting the transformed cell from otherwise lethal chemotherapeutic drugs or drug concentrations. To date, two genes encoding multidrug-resistance export proteins have been identified in the human genome. The first of these, MDR1, encodes P-glycoprotein, a 170 kDa multispanning transmembrane protein belonging the ATP Binding Cassette (ABC) Transporter protein superfamily. Lautier et al. (1996), 52 Biochem. Pharmacol. 967-977. Superfamily members are multispanning tansmembrane proteins that transport substances into or out of the intracellular environment in an energy-dependent manner. Higgins (1992), 8 Ann. Rev. Cell Biol. 67-113, provides a general overview of the properties and natural occurrence of superfamily member proteins. ABC transporters have been identified for a large variety of structurally diverse transported substrates, including sugars, peptides, inorganic ions, amino acids, polysaccharides and proteins. Individual transporter proteins appear to function unidirectionally, i.e., to carry out either export or import of intracellular substances. Thus, P-glycoprotein functions by exporting chemotherapeutic drugs which, although structurally heterogenous, appear to share hydrophobic properties. P-glycoprotein overexpression correlates with the presence of a multidrug-resistance phenotype in diverse tumor cell isolates and tumorigenic cell lines. Significant effort has been invested in the development of agents to block or attenuate P-glycoprotein mediated drug export. Such agents are referred to commonly as "chemosensitizers" or "MDR reversal agents," and are disclosed in Hait et al. (1992), U.S. Pat. No. 5,104,858; Sunkara et al. (1993), U.S. Pat. No. 5,182,293; Sunkara et al. (1993), U.S. Pat. No. 5,190,957; Ramnu et al. (1993), U.S. Pat. No. 5,190,946; Powell et al. (1995), U.S. Pat. No. 5,387,685; Piwnica-Worms (1995), U.S. Pat. No. 5,403,574; Sarkadi et al. (1995), PCT Publication WO 95/31474; Sunkara et al. (1996), U.S. Pat. No. 5,523,304; Zelle et al. (1996), U.S. Pat. No. 5,543,423; Engel et al. (1996), U.S. Pat. No. 5,556,856; Powell et al. (1996), U.S. Pat. No. 5,550,149 and Powell et al. (1996), U.S. Pat. No. 5,561,141. However, P-glycoprotein overexpression does not account for all instances of the acquisition of a multidrug-resistance phenotype. Lautier et al. (1996), 52 Biochem. Pharmacol. 967-977.
A second multidrug-resistance gene identified to date in the human genome encodes multidrug-resistance associated protein (MRP), a 190 kDa multispanning transmembrane protein also belonging to the ABC Transporter protein superfamily. MRP is described in Deeley et al. (1996), U.S. Pat. No. 5,489,519, the teachings of which are incorporated by reference herein. MRP shares only 15% sequence identity with P-glycoprotein at the amino acid level. In addition, MRP differs from P-glycoprotein in its ability to expel specific types of chemotherapeutic drugs from the intracellular milieu. These differences are thought to arise from differences in the drug expulsion mechanism of the two proteins: MRP appears to act on a glutathione-derivatized drug metabolite, whereas P-glycoprotein appears to act on an underivatized drug. Lautier et al. (1996), 52 Biochem. Pharmacol. 967-977. Significantly, agents that block or interfere with P-glycoprotein function appear to have little crossreactivity with MRP. Thus, significant effort is being invested in the development of substances (MDR reversal agents) that block or inhibit MRP function.
Overexpression of either P-glycoprotein or MRP can endow a transformed cell with a multidrug-resistance phenotype; thus, empirical testing is required to determine whether a particular reversal agent will be effective for interfering with a tumor's multidrug resistance phenotype. Currently, it is unclear whether MRP and/or P-glycoprotein expression accounts for all occurrences of the multidrug-resistance phenotype, which arises fairly commonly during the course of chemotherapeutic treatment, irrespective of the tissue specificity of the primary tumor. Moreover, the expression patterns of MRP and P-glycoprotein within a given cell population have been observed to fluctuate over time. Thus, exposure to a reversal agent that interferes with P-glycoprotein function may impose selection pressure favoring the expression of MRP. Such pressure would result in continued viability of cells having a multidrug resistance phenotype. Lautier et al. (1996), 52 Biochem. Pharmacol. 967-977.
Needs remain for preventing or reversing the acquisition of a multidrug resistance phenotype in transformed cells. Particular needs remain to establish the mechanism(s) by which the multidrug resistance phenotype can be produced, and to provide additional therapies for restoring drug sensitivity to multidrug-resistant transformed cells. Still more particular needs remain to improve the clinical management of multidrug resistant tumors, especially when the multidrug resistance phenotype arises entirely or partially from overexpression of one or more genes other than those encoding P-glycoprotein or MRP.